This invention is in the field of glycine. More specifically, this invention is directed to a process for preparing pure or substantially pure glycine.
In the prior art glycine was prepared by; (a) hydrolyzing the nitrile (NH.sub.2 CH.sub.2 CN) with an aqueous alkaline earth metal hydroxide to form an alkaline earth metal salt of the amino acid (glycine); and (b) treating the alkaline earth metal salt with carbon dioxide to form the free amino acid (which remains in solution) and an alkaline earth metal carbonate (which precipitates). The amino acid (glycine) was then recovered. This method, as applied to the preparation of glycine, is taught by U.S. Pat. No. 2,388,189 (Schweitzer, 260/534).
It is desirable to replace the alkaline earth metal hydroxide of the prior art with sodium hydroxide because the latter has a lower equivalent weight than strontium and barium hydroxides, is more soluble than the alkaline earth metal hydroxides, is easier to handle under plant conditions and the ions of sodium, unlike those of barium, (a preferred alkaline earth metal hydroxide) are not toxic. However, such substitution introduces a complication in the separation and recovery of the amino acid (glycine) because sodium carbonate, unlike the alkaline earth metal carbonates, is readily soluble in water, thereby to render the separation and recovery of pure or substantially pure glycine difficult.
A method for separating certain free amino acids from a system comprising the amino acid, sodium chloride, and water is taught by U.S. Pat. No. 3,433,832 (Swanson et al., 260/534).
The Swanson et al. method is not applicable to an amino acid such as glycine which has a solubility greater than 35.0 parts per 100 parts of water at 100.degree. C.
The process of our invention has been found to present an effective and convenient method for recovering glycine from a system consisting essentially of water, glycine and sodium sulfate. Such a system results where glycine is formed from glycinonitrile by hydrolyzing said nitrile with sodium hydroxide and treating the resulting sodium glycinate with sulfuric acid to convert the sodium salt (sodium glycinate) to the free amino acid (glycine). Our resulting aqueous glycine-sodium sulfate solution generally contains at least about 5% glycine, and, if it does not, water can be evaporated therefrom to adjust the glycine concentration thereof to at least about 5% (by weight).
The solid components (glycine or sodium sulfate, respectively) of the slurries formed in the process of our invention can be separated from the respective mother liquors by filtration, decantation, or centrifugation.